The present exemplary embodiment relates to printing systems. It finds particular application in conjunction with scheduling print jobs in print or marking systems with one or more electrophotographic or xerographic print engines. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
The multiple marking engine systems enable high overall outputs to be achieved by printing portions of the same document on multiple printers. Such systems are commonly referred to as “tandem engine” printers, “parallel” printers, or “cluster printing” (in which an electronic print job may be split up for distributed higher productivity printing by different printers, such as separate printing of the color and monochrome pages. Examples of such a system are described below in application Ser. Nos. 10/924,459 and 10/917,768. Such a system feeds paper from a common source to a plurality of printers, which may be horizontally and/or vertically stacked. Printed media from the various printers is then taken from the printers to a finisher where the sheets associated with a single print job are assembled.
Typically, printing systems operate by serially processing sheets from jobs that are in a serial job queue, where the order of the jobs in the queue is the order in which the jobs were submitted. As is known by product developers and operators of such systems, certain classes of jobs stress the print engine and lead to poor performance or failures. Printing an extended job with particularly high or low area coverage can create wear or charging distribution problems in the developer sump, subsequently leading to problems such as high background, bead carry out, drift in developed mass per area, and so on. Stress jobs are often said to be outside of the operational latitude of the system which leads to a reduced performance of the print engines. While countermeasures are generally known for recovering from the performance shortfalls or failures associated with the operation outside of the print system's latitude, there is usually collateral waste or loss in productivity. In some circumstances, closed loop controls may be able to open system latitude or maintain system performance. However, this approach may be costly or even non-viable to develop, and may decrease productivity or increase waste when implemented.
One example of a stress causing job is one where the long term average toner consumption per page is low for a monochrome engine or for any color of a process color engine. This typically occurs when a print job has a preponderance of low area coverage monochrome pages or a preponderance of low area coverage images for one or more of the color separations for process color pages. If too low of an average consumption rate for any toner persists for too many pages, the marking materials are not used at a sufficient rate, and the supply is not regularly replenished with the fresh material. Over an extended period, the marking material stored in the developer housing becomes damaged due to the constant churning of the material under high shear. Examples of damage are the impaction of toner particles onto carrier beads, the impaction of additives onto toner, and the degradation of carrier bead coatings. Surface charge distribution of materials damaged in the developer housing can become skewed or pathologically abnormal. Images printed with the damaged material will have one or more image defects, such as color imbalances, fine line growth or shrinkage, or high levels of background toner in the nominally white region of a page, which can appears as a color shift or dirt over the entire printable area of the page. A wasteful countermeasure to printing low area coverage jobs is to purge significant quantities of damaged toner from the developer sump. Fresh toner or toner with small amounts of carrier can then be dispensed into the developer sump to restore the charging performance of the marking materials.
There is a need for methods and apparatuses that overcome the aforementioned problems and others.